The present invention relates to an optical element and a manufacturing method thereof and, more particularly, relates to an optical element capable of recording a stereoscopic image as a hologram and reconstructing the image, and a manufacturing method thereof.
A holographic technique is conventionally known as a method for recording a stereoscopic image on a medium and reconstructing this image. A hologram produced by this method is used in various fields, such as ornamental art or anti-counterfeit seals. In order to optically produce the hologram, it is common to record the interference fringe between object light reflected from an object and reference light on a photosensitive medium. A laser beam superior in coherence is usually used as a light source for the object light and the reference light. Generally, the motion of electromagnetic radiation, such as light, can be regarded as the propagation of a wave front provided with amplitude and a phase, and it can be said that the hologram is an optical element that functions to reconstruct such a wave front. Therefore, it is necessary to record information for accurately reconstructing the amplitude and phase of the object light at each position in space on the recording medium of the hologram. If interference fringes generated by the object light and the reference light are recorded on the photosensitive medium, information that includes both the phase and the amplitude of the object light can be recorded, and, by projecting illumination reconstructing light equivalent to the reference light onto the medium, a part of the illumination reconstructing light can be observed as light provided with a wave front equivalent to the object light.
If the hologram is produced by an optical method using a laser beam or the like in this way, the phase and amplitude of the object light can be recorded only as interference fringes resulting from interference between the object light and the reference light. The reason is that the photosensitive medium has a property of being photosensitized in accordance with light intensity. On the other hand, a technique of producing a hologram by computations with use of a computer has recently been put to practical use. This technique is called a xe2x80x9cCGHxe2x80x9d (Computer-Generated Hologram) method, in which the wave front of object light is calculated by use of a computer, and its phase and its amplitude are recorded on a physical medium according to a certain method so as to produce a hologram. The employment of this computational holography, of course, enables the recording of an image as interference fringes between object light and reference light, and, in addition, enables the recording of information for the phase and amplitude of the object light directly onto a recording surface without using the reference light. For example, a recording method has been proposed in which an amplitude is represented by the size of an opening formed in a recording medium whereas a phase is represented by the position of the opening or in which a medium is made up of two recording layers on one of which an amplitude is recorded and on the other one of which a phase is recorded.
The method for recording an image as interference fringes that has been widely used as an optical hologram producing method is at an advantage in that productivity is high because, in general, a reconstructed image with high resolution can be obtained and because an optical method is used, but it is at a disadvantage in that an image darkens because diffraction efficiency by interference fringes is poor when reconstructed. By contrast, the method for recording the phase and amplitude of object light directly onto a medium that has been proposed as one of the computer-generated hologram methods is at an advantage in that high diffraction efficiency can be obtained, but it is at a disadvantage in that, practically, productivity decreases because the recording of the phase and the amplitude onto the medium is technically difficult.
It is therefore an object of the present invention to provide an optical element that can obtain high diffraction efficiency when reconstructed and that is excellent in productivity.
(1) The first feature of the present invention resides in an optical element consisting of a set of a plurality of three-dimensional cells, wherein:
a specific amplitude and a specific phase are defined in each individual cell,
and the individual cell has a specific optical property so that, when incident light is provided to the cell, emission light is obtained by changing an amplitude and a phase of the incident light in accordance with the specific amplitude and the specific phase defined in the cell.
(2) The second feature of the present invention resides in the optical element according to the first feature, wherein each cell has an amplitude-modulating part provided with transmittance corresponding to a specific amplitude.
(3) The third feature of the present invention resides in the optical element according to the first feature, wherein each cell has an amplitude-modulating part provided with reflectivity corresponding to a specific amplitude.
(4) The fourth feature of the present invention resides in the optical element according to the first feature, wherein each cell has an amplitude-modulating part provided with an effective area corresponding to a specific amplitude.
(5) The fifth feature of the present invention resides in the optical element according to the first to the fourth features, wherein each cell has a phase-modulating part provided with a refractive index corresponding to a specific phase.
(6) The sixth feature of the present invention resides in the optical element according to the first to the fourth features, wherein each cell has a phase-modulating part provided with an optical path length corresponding to a specific phase.
(7) The seventh feature of the present invention resides in the optical element according to the first feature, wherein each cell has a concave part formed by hollowing a part provided with an area corresponding to a specific amplitude by a depth corresponding to a specific phase.
(8) The eighth feature of the present invention resides in the optical element according to the first feature, wherein each cell has a convex part formed by protruding a part provided with an area corresponding to a specific amplitude by a height corresponding to a specific phase.
(9) The ninth feature of the present invention resides in the optical element according to the seventh or eighth feature, wherein a surface where the concave part or the convex part of each cell is formed serves as a reflecting surface, and incident light provided to the cell is reflected by the reflecting surface and thereby turns into emission light.
(10) The tenth feature of the present invention resides in the optical element according to the seventh or eighth feature, wherein each cell includes a main body layer having a concave part or a convex part and a protective layer with which a surface where the concave part or the convex part of the main body layer is formed is covered, and the main body layer and the protective layer are made of materials different from each other.
(11) The eleventh feature of the present invention resides in the optical element according to the tenth feature, wherein the main body layer and the protective layer are made of transparent materials different in a refractive index from each other, and incident light provided to the cell passes through the main body layer and the protective layer and thereby turns into emission light.
(12) The twelfth feature of the present invention resides in the optical element according to the tenth feature, wherein a boundary between the main body layer and the protective layer forms a reflecting surface, and incident light provided to the cell is reflected by the reflecting surface and thereby turns into emission light.
(13) The thirteenth feature of the present invention resides in the optical element according to the first to the twelfth features, wherein each cell is arranged one-dimensionally or two-dimensionally.
(14) The fourteenth feature of the present invention resides in the optical element according to the thirteenth feature, wherein a longitudinal pitch of each cell and a lateral pitch of each cell are arranged so as to be an equal pitch.
(15) The fifteenth feature of the present invention resides in the optical element according to the first to the fourteenth features, wherein a complex amplitude distribution of object light from an object image is recorded so that the object image is reconstructed when observed from a predetermined viewing point so as to be usable as a hologram.
(16) The sixteenth feature of the present invention resides in a method for manufacturing an optical element where a predetermined object image is recorded, the method comprising:
a cell defining step of defining a set of a plurality of three-dimensional virtual cells;
a representative-point defining step of defining a representative point for each virtual cell;
an object image defining step of defining an object image to be recorded;
an amplitude phase defining step of defining a specific amplitude and a specific phase in each virtual cell by calculating a complex amplitude at a position of each representative point of object light emitted from the object image; and
a physical cell forming step of replacing each virtual cell with a real physical cell and forming an optical element that consists of a set of three-dimensional physical cells;
wherein, at the physical cell forming step, when predetermined incident light is given to each physical cell, replacement is carried out by each physical cell having a specific optical property so as to obtain emission light that has changed an amplitude and a phase of the incident light in accordance with a specific amplitude and a specific phase defined in the virtual cell corresponding to the physical cell.
(17) The seventeenth feature of the present invention resides in the manufacturing method for the optical element according to the sixteenth feature, wherein at the cell defining step, a cell set is defined by arranging block-like virtual cells one-dimensionally or two-dimensionally.
(18) The eighteenth feature of the present invention resides in the manufacturing method for the optical element according to the sixteenth or seventeenth feature, wherein at the amplitude phase defining step, a plurality of point light sources are defined on the object image, and object light of a spherical wave having a predetermined amplitude and a predetermined phase is regarded as being emitted from each point light source, and a totaled complex amplitude of the object light from the point light sources at a position of each representative point is calculated at a predetermined standard time.
(19) The nineteenth feature of the present invention resides in the manufacturing method for the optical element according to the eighteenth feature, wherein K point light sources that emit object light whose wavelength is xcex are defined on the object image, and if an amplitude of object light emitted from a k-th point light source O(k) (k=1 to K) is represented as Ak, and a phase thereof is represented as xcex8k, and a distance between a predetermined representative point P and the k-th point light source O(k) is represented as rk, a totaled complex amplitude of the object light from the K point light sources at the predetermined representative point P is calculated as follows:
xcexa3(k=1,K)(Ak/rkxc2x7cos(xcex8kxc2x12xcfx80rk/xcex)+iAk/rkxc2x7sin(xcex8kxc2x12xcfx80rk/xcex)). 
(20) The twentieth feature of the present invention resides in the manufacturing method for the optical element according to the sixteenth to nineteenth features, wherein, at the physical cell forming step, each virtual cell is replaced with a physical cell having a concave part formed by hollowing a part provided with an area corresponding to a specific amplitude by a depth corresponding to a specific phase.
(21) The twenty-first feature of the present invention resides in the manufacturing method for the optical element according to the sixteenth to nineteenth features, wherein, at the physical cell forming step, each virtual cell is replaced with a physical cell having a convex part formed by protruding a part provided with an area corresponding to a specific amplitude by a height corresponding to a specific phase.
(22) The twenty-second feature of the present invention resides in the manufacturing method for the optical element according to the twentieth or twenty-first feature, wherein:
a refractive index of a material filled in the concave part of the physical cell or a material that constitutes the convex part is represented as n1,
a refractive index of another material in contact with the material n1 is represented as n2,
a wavelength of object light is represented as xcex,
a maximum depth dmax of the concave part or a maximum height dmax of the convex part is set to be dmax=xcex/|n1xe2x88x92n2|,
a depth or height d corresponding to a specific phase xcex8 is determined by the expression d=xcexxc2x7xcex8/2(n1xe2x88x92n2)xcfx80 when n1 greater than n2, and is determined by the expression d=dmaxxe2x88x92xcexxc2x7xcex8/2(n2xe2x88x92n1)xcfx80 when n1 less than n2, and
an object image is reconstructed by transmission light that has passed through the concave part or the convex part.
(23) The twenty-third feature of the present invention resides in the manufacturing method for the optical element according to the twentieth or twenty-first feature, wherein:
a refractive index of a material filled in the concave part of the physical cell or a material that constitutes the convex part is represented as n, a wavelength of object light is represented as xcex,
a maximum depth of the concave part or a maximum height dmax of the convex part is set to be dmax=xcex/2n,
a depth or a height d corresponding to the specific phase xcex8 is determined by the expression
d=xcexxc2x7xcex8/4nxcfx80, 
and
an object image is reconstructed by reflected light that has been reflected by the boundary of the concave part or the convex part.
(24) The twenty-fourth feature of the present invention resides in the manufacturing method for the optical element according to the twentieth to twenty-third features, wherein xcex1 kinds of a plurality of areas are defined as areas corresponding to a specific amplitude, xcex2 kinds of a plurality of depths or heights are defined as depths or heights corresponding to a specific phase so as to prepare xcex1xc3x97xcex2 kinds of physical cells in total, and each virtual cell is replaced with a physical cell closest in a necessary optical property among said physical cells.
(25) The twenty-fifth feature of the present invention resides in the manufacturing method for the optical element according to the sixteenth to twenty-fourth features, further comprising a phase-correcting step of correcting the specific phase defined for each virtual cell in consideration of a direction of illumination light projected when reconstructed or in consideration of a position of a viewing point when reconstructed.
(26) The twenty-fifth feature of the present invention resides in the manufacturing method for the optical element according to the sixteenth to twenty-fifth features, wherein:
at the cell defining step, a cell set of virtual cells arranged on a two-dimensional matrix is defined by arranging the virtual cells horizontally and vertically,
at the amplitude phase defining step, a plurality of M point light source rows that are each extended in a horizontal direction and are mutually disposed in a vertical direction are defined on an object image, and M groups in total are defined by defining virtual cells that belong to a plurality of rows contiguous in the vertical direction in the two-dimensional matrix as one group,
the M point light source rows and the M groups are caused to correspond to each other in accordance with an arrangement order relative to the vertical direction, and
a totaled complex amplitude at a position of each representative point is calculated on a supposition that object light emitted from a point light source in an m-th point light source row (m=1 to M) reaches only virtual cells that belongs to an m-th group.